1. Field of the Invention
The present invention relates to a transversely stretched nonwoven fabric of large width manufactured with transverse stretch of stretching a web in its transverse direction as one of so-called post stretch methods for nonwoven fabrics in which a web formed from spun filaments is stretched in manufacturing a nonwoven fabric. The transversely stretched nonwoven fabric is used as a nonwoven fabric requiring strength in a transverse direction and as a material web for an orthogonal nonwoven fabric, or as a nonwoven fabric with strength and dimensional stability.
2. Description of the Related Art
Methods of manufacturing nonwoven fabrics include a spun-bonding scheme, a melt blow scheme, a spun lace scheme which form a nonwoven fabric directly from spinning of a melting material resin, and the like. These schemes are dominant in the nonwoven fabric manufacturing methods in terms of economics and manufacturablility. Nonwoven fabrics manufactured with these schemes are hereinafter referred to as spun bonded nonwoven fabrics in a broad sense. The spun bonded nonwoven fabrics in a broad sense with the prior art are random nonwoven fabrics which have disadvantages that strength is low and they often have no dimensional stability.
Methods and apparatuses for manufacturing nonwoven fabrics which improve the aforementioned disadvantages are described in Japanese Patent Publication No.36948/91, Japanese Patent Publication No.6126/95 and Japanese Patent No.2612203 by the present applicant.
Japanese Patent Publication No.36948/91 describes, as a method of manufacturing a nonwoven fabric, a method of stretching a long fiber nonwoven fabric formed by spinning un-oriented filaments in one direction at a stretch suitable temperature such that many of the filaments are aligned in one direction. The gazette describes a method of laminating and bonding nonwoven fabrics stretched in such a method such that the respective stretch directions of the nonwoven fabrics are orthogonal to each other.
In addition, the aforementioned gazette describes, as spray spinning, a method of manufacturing a long fiber nonwoven fabric comprising un-oriented filaments aligned in one direction. In the method of manufacturing a long fiber nonwoven fabric, first, filaments pushed out from a nozzle are scattered by heated air which rotates in a spiral over a screen mesh running in one direction. Besides the rotating air, two flows of air are jetted such that they collide with each other below the nozzle. The air spread by the colliding two flows of air further scatters the rotating spun filaments. When the directions of the two flows of air jetted such that they collide with each other are parallel to the running direction of the screen mesh, the spun filaments are scattered in a direction perpendicular to the running direction of the screen mesh. This causes the scattered filaments to be accumulated on the screen mesh with many of the components being aligned transversely, thereby manufacturing a nonwoven fabric with the filaments mainly aligned transversely. On the other hand, when the directions of the two flows of air jetted such that they collide with each other are substantially orthogonal to the running direction of the screen mesh, the spun filaments are scattered in a direction parallel to the running direction of the screen mesh. This causes the scattered filaments to be accumulated on the screen mesh with many of the components being aligned longitudinally, thereby manufacturing a nonwoven fabric with the filaments mainly aligned longitudinally.
Japanese Patent Publication No.6126/95 describes, as spray spinning, a method of manufacturing a nonwoven fabric with one direction alignment in which a plurality of filaments are aligned substantially in one direction. In the manufacturing method, when filaments are spun by discharging polymeric materials from a port for spinning, first, the spun filaments are rotated or vibrated in a width direction. While the rotating or vibrating filaments have draft properties by a factor of two or more, the filaments are acted on by a pair of fluids or more substantially symmetrically about one of the rotating or vibrating filaments from the side of the filament. xe2x80x9cDraft propertiesxe2x80x9d refers to the property of how much filaments are drawn. Such action of the pair of fluids or more on the filaments causes the filaments to be scattered in a direction perpendicular to the discharged direction of the filaments while draft is applied to the filaments. As a result, the filaments aligned in the scattered direction thereof are laminated in layered form to manufacture a nonwoven fabric with one direction alignment comprising the laminated filaments. xe2x80x9cDraft is applied to the filamentsxe2x80x9d refers to drawing and thinning of the filaments by applying traction to them.
Japanese Patent No.2612203 describes, as a method of manufacturing a nonwoven fabric, a method of manufacturing a web made of fibers aligned in one direction. In the manufacturing method, fibers are jetted together with a fluid from an ejector onto a running belt conveyor, and the fibers are collected on the belt conveyor such that the fibers are aligned in one direction. In an example of such a manufacturing method, at least part of the conveyor belt is curved perpendicularly to its running direction and downward such that the fluid and the fibers are jetted from the ejector toward the bottom of the curved portion in groove shape in the conveyor belt. The jetted fluid is scattered in a longitudinal direction of the groove in the conveyor belt, thereby aligning the fibers in the scattered direction.
In manufacturing the nonwoven fabrics, manufacture of a web of large width with high strength of a nonwoven fabric maintained,is an important factor since not only does it mean an increase in production efficiency and a reduction in manufacturing unit cost, but also in terms of applications of the web, some fields find no applicability unless a web of large width is used.
Since a transversely stretched web is manufactured by stretching an original web in its transverse direction, it is easily obtained as a web of large width in general. In the spray spinning described in the aforementioned Japanese Patent Publication No.36948/91 and Japanese Patent Publication No.6126/95, since an original web before stretch typically has a width of 300 to 400 mm, the transverse stretch rate of the web is 5 to 6 for a web made of polypropylene (hereinafter referred to as xe2x80x9cPPxe2x80x9d), or 5 to 6 for a web made of polyethylene terephthalate (hereinafter referred to as xe2x80x9cPETxe2x80x9d). Therefore, there is a problem of the difficulty in realizing a transversely stretched nonwoven fabric with a width of 2400 mm or more as a product. A web of large width with a higher stretch rate can be manufactured by employing spinning conditions for increasing the diameter of filaments of an original web when the original web is manufactured with the spray spinning described in Japanese Patent Publication No.36948/91 and Japanese Patent Publication No.6126/95, and in this case more stable spinning is possible.
Conventionally, means for transversely stretching a web with both edge portions thereof held is generally used as a transverse stretch apparatus for stretching a web in its transverse direction. In addition, as such a transverse stretch apparatus, a tenter frame for use in stretching a film transversely can be used. Simple transverse stretch apparatuses are a pulley type transverse stretch apparatus, and for a film, an example is described in GB Patent Specification No.1213441. An application of the apparatus to a nonwoven fabric is described in Japanese Patent Publication No.3068/88, Japanese Patent Publication No.36948/91 (U.S. Pat. No. 4,992,124 Specification corresponding to that gazette) which are the earlier inventions by the present inventors. A transverse stretch apparatus with means composed by combining a pair of upper and lower groove rolls has been used (Japanese Patent Publication No.32307/84, U.S. Pat. No. 4,223,059 Specification).
In general, a transversely stretched web of large width can be obtained from an original web of small width by increasing a transverse stretch rate. However, since stretch at a higher rate involves stretch breaking of filaments at the stretch, the rate inevitably has a limitation. While it is possible to achieve only an increase in the stretch rate by increasing the temperature of an original web or the stretch temperature in stretching the original web, stretch at a high temperature cannot ensure sufficient strength of the resultant transversely stretched web in general. Thus, a stretch rate of the order of 5 to 6 as described above can ensure a certain degree of strength, but at a stretch rate of 7 or more, it is difficult to manufacture a transversely stretched web of large width with a desired strength.
On the other hand, as a method of stretching a web in its lengthwise direction or a longitudinal direction there is a proximity stretch method. The proximity stretch is a method in which rollers are used for longitudinally stretching a web, for example, to perform a longitudinal stretch with an extremely reduced stretch distance for the web. It is difficult to apply the proximity stretch which is applied to such longitudinal stretch to transverse stretch of a web. In the transverse stretch, generally, an original web is transversely stretched with both edge portions thereof held. Therefore, it is desirable not only to transversely align filaments constituting a web but also to extend each filament from one edge to the other edge in a width direction of the web in the transverse stretch of webs as compared with the longitudinal stretch. Thus, the proximity stretch used in the longitudinal stretch can not be applied to the transverse stretch, and the transverse stretch methods and transverse stretch apparatuses in the prior art have a problem of the difficulty in realizing a high stretch rate with the high strength of a web being maintained.
In the transverse stretch, as a mechanism similar to the proximity stretch using rolls in the longitudinal stretch, Japanese Patent Publication No.36948/91 describes a transverse stretch method of groove roll type, for example. The transverse stretch method of groove roll type employs a pair of groove rolls which is arranged such that peaks of one groove roll match valleys of the other groove roll. A nonwoven fabric comprising un-oriented filaments is introduced between the pair of groove rolls to transversely stretch the nonwoven fabric by means of the projections and depressions of the peaks and valleys of the groove rolls. However, the transverse stretch method of groove roll type has disadvantages of a low stretch rate, low uniformity of stretch and the like, which render the method unsuitable as a stretch method for obtaining high strength of a nonwoven fabric. As a result, the method can be used for stretch of a nonwoven fabric which does not require high strength, but it is not suitable for obtaining a high stretch rate and high strength.
In particular, the stretch of PET filaments has characteristics of a narrow range of temperatures suitable for stretch at which high strength of a nonwoven fabric is obtained, i.e., stretch suitable temperatures at which high strength of a nonwoven fabric is obtained, and significant variations in the stretch suitable temperature depending on a stretch speed and a stretch rate. These characteristics make it difficult to stretch a web comprising PET filaments. Specifically, it is difficult to obtain a transversely stretched web of large width with high strength maintained and at a high stretch rate as a web comprising PET filaments. Therefore, stretch which can solve such difficulties must be performed in order to manufacture a web of large width comprising PET filaments.
In such transverse stretch at a high rate, it is necessary not only to achieve a high stretch rate, but also to provide uniformly stretched portions of a transversely stretched web obtained by stretch and to provide uniform strength distribution and basis weight in the stretched web, a reduced frequency of stretch breaking of filaments and the like. Therefore, a method of transversely stretching a web at a high stretch rate does not provide stretch means in an industrial sense unless such uniform transverse stretch is realized.
When a transverse stretch apparatus used for the aforementioned purposes is expensive or requires a large floor area, the transverse stretch apparatus has no practicality as a nonwoven fabric manufacturing apparatus which has the requirement of being inexpensive. In addition, it is necessary for the transverse stretch apparatus to freely change a stretch rate and to easily deal with troubles such as stretch breaking in a simple manner. Furthermore, even such a simple and inexpensive transverse stretch apparatus must enable fast stretch and realize uniform stretch at a high rate as described above. Particularly in stretch of a nonwoven fabric, the aforementioned purposes such as stretch at a high rate, high-speed stretch, and uniform stretch cannot be achieved unless an original material formed from collected flocculent filaments and an apparatus are employed in the stretch, the apparatus being capable of completely replacing the air contained within the collected flocculent filaments with a heated medium at a temperature increased to a stretch temperature.
It is an object of the present invention to provide a transversely stretched nonwoven fabric in which, when an original web is transversely stretched to manufacture a transversely stretched nonwoven fabric, high tensile strength can be obtained in the resultant nonwoven fabric in a transverse direction even at a stretch rate of the original web of 7 or higher, specifically, at least 132.5 mN/tex (1.5 g/d), desirably 158.9 mN/tex (1.8 g/d) or higher, more desirably 176.6 mN/tex (2.0 g/d) or higher, most desirably 220.8 mN/tex (2.5 g/d) or higher is obtained as a tensile strength of the web in the transverse direction, a method of manufacturing such a transversely stretched nonwoven fabric, and a transverse stretch apparatus capable of manufacturing a transversely stretched nonwoven fabric at such a stretch rate and strength.
It is another object of the present invention to provide a transversely stretched nonwoven fabric having texture like a cloth, a method of manufacturing such a transversely stretched nonwoven fabric, and a transverse stretch apparatus capable of manufacturing such a transversely stretch nonwoven fabric. It is desired that the diameter of filaments constituting the stretched web is at least 20 xcexcm or lower, desirably 10 xcexcm, more desirably 5 to 8 xcexcm.
It is a further object of the present invention to provide a transverse stretch apparatus which, when an original web is heated in order to transversely stretch the original web to manufacture a transversely stretched nonwoven fabric, allows quick and uniform heating of the original web to provide fast and uniform stretch of the original web at a high rate, and a heating unit for use in the transverse stretch apparatus.
To achieve the aforementioned objects, the transversely stretched nonwoven fabric according to the present invention comprises a plurality of transversely aligned filaments with a fiber diameter of 20 xcexcm or lower, has a stretch rate of 7 or more in a transverse direction, and has a tensile strength of 132.5 mN/tex (1.5 g/d) or higher in the transverse direction. For the tensile strength of a transversely stretched nonwoven fabric, breaking strength is represented as a breaking load per 5 centimeters in the long fiber filament nonwoven fabric test method in compliance with JIS (Japanese Industrial Standards) L1906. However, the present invention employs representation of tensile strength as strength per tex (mN/tex) with conversion from the weight of a nonwoven fabric to fineness (tex) since nonwoven fabrics of various basis weights have been tested. For reference, strength per denier (d) is also shown in the following description. The realization of the aforementioned transversely stretched nonwoven fabric can result in a web of large width at a stretch rate of 7 or more in the transverse direction while a high tensile strength of 132.5 mN/tex (1.5 g/d) or higher is maintained. This causes enhanced production efficiency of the transversely stretched nonwoven fabric and reduced manufacturing unit cost of the transversely stretched nonwoven fabric as a web of large width. In addition, such a transversely stretched nonwoven fabric with high strength and a high transverse stretch rate obtained as a web of large width leads to wide applicability of the transversely stretched nonwoven fabric in terms of applications of the web. Moreover, since the fiber diameter of the filaments constituting the transversely stretched nonwoven fabric is 20 xcexcm or lower, the transversely stretched nonwoven fabric has the texture of cloth.
In the method of manufacturing the transversely stretched nonwoven fabric according to the present invention, first, an original web comprising un-oriented filaments is stretched 1.2 to 3 times wider in its transverse direction at a temperature higher than its stretch suitable temperature by 5xc2x0 C. or more. This step causes the filaments of the original web to be stretched with almost no molecular orientation of the filaments involved. At this point, the strength of the original web is not increased yet. If the stretch rate in the transverse direction is 1.2 or lower at this step, the original web cannot be transversely stretched at a high rate at the next step, and if the stretch rate is 3 or higher, the strength of the original web is reduced. Next, the original web stretched 1.2 to 3 times wider in the transverse direction is further stretched transversely at the stretch suitable temperature to stretch the original web 7 times wider or more in the transverse direction in total as compared with the state of the original web before the stretch. In this manner, a transversely stretched nonwoven fabric made of the original web stretched 7 times wider or more in the transverse direction in total is manufactured. At this step, the original web is transversely stretched at a high rate at the stretch suitable temperature of the original web, and it is possible to obtain a tensile strength of the transversely stretched nonwoven fabric in the transverse direction equal to or higher than that in normal stretch at the stretch suitable temperature. The method of manufacturing the transversely stretched nonwoven fabric including the two-step stretch provides a web of large width stretched at a high rate of 7 or more while a high tensile strength of 132.5 mN/tex (1.5 g/d) or higher in the transverse direction is maintained. Thus, the production efficiency of the transversely stretched nonwoven fabric is increased, and manufacturing unit cost of the transversely stretched nonwoven fabric as a web of large width is reduced. It is also possible to manufacture such a transversely stretched nonwoven fabric as a web of large width with high strength and a high transverse stretch rate and to obtain a transversely stretched nonwoven fabric with wide applicability in terms of applications of the web.
The stretch suitable temperature depends on kinds of polymers of filaments, degree of polymerization, temperature of spinning, speed of spinning, cooling condition and so on. In general, a stretch temperature at which strength of the web after stretching rises most when a nonwoven fabric is stretched is adopted as the stretch suitable temperature. A range of the stretch suitable temperature of polypropylene nonwoven fabric which is well-cooled in spinning is between 100xc2x0 C. to 130xc2x0 C. in the hot wind stretching, preferably between 105xc2x0 C. to 120xc2x0 C. The stretch suitable temperature of polyethylene telephthalate nonwoven fabric depends on a stretch speed mainly, the stretch suitable temperature is 80xc2x0 C. to 95xc2x0 C. at low speed and 95xc2x0 C. to 105xc2x0 C. at high speed.
xe2x80x9cunoriented filamentxe2x80x9d does not mean that the degree of molecular orientation is completely zero, it is possible to stretch the unoriented filament 3 times wider or more at its stretch suitable temperature. When unoriented filament is shown with degree of molecular orientation which was measured with birefringence and so on, degree of orientation of a perfect orientation is 100% and xe2x80x9cunoriented filamentxe2x80x9d refers to the filament having degree of orientation of 10% or less.
According to one aspect of the method of manufacturing a transversely stretched nonwoven fabric of the present invention, a method of manufacturing a transversely stretched nonwoven fabric particularly suitable for a web comprising un-oriented filaments mainly made of polyethylene terephthalate (PET) is provided as a manufacturing method which allows the web to be stretched transversely at a high stretch rate. In the manufacturing method, while an original web comprising un-oriented filaments mainly made of polyethylene terephthalate is heated by hot air to a temperature of 100xc2x0 C. or higher and is moved at a line speed of 20 m/min or higher at the same time, the original web is stretched 7 times wider or more in its transverse direction in total such that the original web has a tensile strength of 132.5 mN/tex (1.5 g/d) in a transverse direction. According to the manufacturing method, when a transversely stretched nonwoven fabric mainly made of PET is manufactured, it is possible to manufacture a transversely stretched nonwoven fabric with a tensile strength of 132.5 mN/tex (1.5 g/d) in the transverse direction and a transverse stretch rate of 7 or more. It is preferable to use an original web comprising transversely aligned filaments as the original web before the transverse stretch. The use of such an original web for manufacturing the transversely stretched nonwoven fabric enables high strength and a high stretch rate to be realized in the transversely stretched nonwoven fabric.
The transverse stretch apparatus of the present invention uses the method of manufacturing the transversely stretch nonwoven fabric with the two-step stretch as described above. In the transverse stretch apparatus, first, an original web comprising un-oriented filaments is heated by first heating means to a temperature higher than its stretch suitable temperature by 5xc2x0 C. or more. The heated original web is stretched 1.2 to 3 times wider in its transverse direction by first stretch means. This causes the filaments of the original web to be stretched with almost no molecular orientation of the filaments involved. At this point, the strength of the original web is not increased yet. Next, the original web stretched 1.2 to 3 times wider in the transverse direction is heated to the stretch suitable temperature by second heating means, and is further stretched transversely by second stretch means at the stretch suitable temperature. This results in a transversely stretched nonwoven fabric made of the original web stretched 7 times wider or more in the transverse direction in total as compared with the state before the stretch by the first stretch means. Such a transverse stretch apparatus can be used to transversely stretch the original web at a high rate at the stretch suitable temperature thereof, and to obtain a tensile strength of the transversely stretched nonwoven fabric in the transverse direction equal to or higher than that in normal stretch at the stretch suitable temperature. The transverse stretch apparatus of performing such two-stretch provides, as a web of large width, a transversely stretched nonwoven fabric stretched at a high rate of 7 or more while a high tensile strength of 132.5 mN/tex (1.5 g/d) or higher in the transverse direction is maintained. Thus, the production efficiency of the transversely stretched nonwoven fabric is increased, and manufacturing unit cost of the transversely stretched nonwoven fabric as a web of large web is reduced. Since such a transversely stretched nonwoven fabric can be manufactured as a web of large width with high strength and a high transverse stretch rate, it is also possible to obtain a transversely stretched nonwoven fabric with wide applicability in terms of applications of the web.
In addition, first preheat means may be provided for preheating the original web by blowing hot air toward the original web such that the hot air passes through the original web before the heating of the original web by the first heating unit.
Second preheat means may also be provided for heating the original web by blowing hot air toward the original web such that the hot air passes through the original web before the original web stretched transversely by the first stretch means is heated by the second heating unit to its stretch suitable temperature.
The heating unit of the present invention heats the original web by hot air before the original web is transversely stretched and has a net member for supporting the original web at the heating by the hot air. The heating unit is provided, for example, for a transverse stretch apparatus for manufacturing a transversely stretched nonwoven fabric by transversely stretching an original web. When the original web is heated by the heating unit, hot air is blown toward the original web such that the hot air passes through the original web. At this point, the original web is supported on the net member by said net member contacting the surface of the original web opposite to the side on which the hot air is blown in the portion in which the hot air is blown. At least part of the hot air through the original web further passes through the net member. Thus, deformation of the original web such as expansion thereof due to the hot air is prevented. Since at least part of the hot air through the original web further passes through the net member, the air contained in the original web, or the air present in the gaps between the filaments forming the original web is replaced with the air at a high temperature for heating the original web. Thus, the original web can be quickly and uniformly heated to an intended stretch temperature to stretch the original web. As the net member, a mesh roll of hollow cylindrical shape rotatably supported and having a plurality of through holes formed in its wall, or a mesh conveyor belt supported movably in one direction can be used.
The above and other objects, features and advantages of the present invention will become apparent from the following description with reference to the accompanying drawings which illustrate examples of the present invention.